Aqueous polyacrylic dispersions are aqueous dispersions of acrylic and methacrylic esters having a particle size in the range of about 0.05 micron up to about 0.5 micron. They are useful in the manufacture of latex paints, water-based inks, paper coatings, water-borne adhesives (such as pressure-sensitive adhesives), and as binders for nonwoven materials.
United Kingdom Patent Specification 1,278,813 indicates that it is known that stable dispersions or lattices can be obtained by the emulsion polymerization of acrylic or methacrylic esters in the presence of over 25%, based on the weight of the monomer, of vinyl acetate and a protective colloid such as hydroxyethyl cellulose (HEC). When less than 25% of the vinyl acetate comonomer is used, a stable latex cannot be obtained if the protective colloid is present.
Further, it is known, for instance, from U.S. Pat. No. 4,265,796, that the presence of protective colloids during the emulsion polymerization of acrylic esters frequently results in coagulated dispersions or in extremely viscous dispersions which are unsuitable for use as, e.g., paint binders.
The use of protective colloids has been avoided by using high levels of surfactants, e.g., about 5% to about 10%, based on the weight of monomer, in place of the protective colloid. However, the properties of the resultant latex, such as its film-forming ability, tensile strength, film-blocking resistance and water and solvent resistance are adversely affected.
British Patent Specification No. 787,703 discloses a process for preparing acrylic lattices by emulsion polymerization of acrylic and methacrylic esters in the presence of, among other things, sodium styrene sulfonate. The need for a protective colloid is avoided by the formation of an aqueous latex of an interpolymer formed from an alkyl ester of acrylic or methacrylic acid, acrylonitrile and/or methacrylonitrile, acrylic acid or a monomethyl or monophenyl acrylic acid, and a monovinylidene aromatic hydrocarbon or a nuclear halogen substitution product thereof. The disclosed process is extremely complex and subject to many specific and critical procedural restrictions.
Thus, there is a need for an aqueous acrylic polymer dispersion which can be prepared by a simple, yet effective emulsion polymerization process, which does not require the presence of protective colloids or high surfactant levels. Further, there is a need for a simple and effective process for the emulsion polymerization of acrylic and methacrylic esters in the absence of a protective colloid, and that does not require the presence of high surfactant levels.